Thirty-seven strains of viridans streptococci isolated from blood cultures on bone marrow transplant (BMT) recipients during the past 4 years and 39 strains of viridans streptococci isolated from non-BMT patients were tested for fluoroquinolone susceptibility. Of the five strains from BMT patients that were resistant to levofloxacin, trovafloxacin, and/or grepafloxacin, one isolate (NIH-(R02) demonstrated high-level resistance to multiple fluoroquinolones tested. DNA gyrase A is one of the major targets for quinolone antibiotics. The amino acid sequence within the quinolone resistance-determining region (QRDR) of the gyrase A of NIH-(R02) was determined. When compared with ten sensitive strains of S. mitis, NIH-(R02) carried a single point mutation in the "GKYHPHGDS" box within the QRDR of the gyrA gene (serine to leucine replacement). The mechanism of fluoroquinolone resistance in this isolate of S. mitis is thus likely similar to the previously reported mechanisms in S. pneumoniae.We will sequence the QRDR of the parC subunit of topoisomerase IV (parC gene) of NIH-(R02) and the ten sensitive strains to search for additional point mutations that may confer resistance to the quinolone antibiotics in the viridans group of streptococci.